The present invention relates to a characterisation or control method for preparation of a thin layer component by optical methods.
For the production of high quality optical layers, increased control of the preparation of each layer and of its refraction index has become a crucial challenge. Among the different control method, it is well known that ellipsometry is one of the most sensitive. It has therefore been contemplated to compare in real time the evolution of ellipsometric parameters Is, Ic, or any other combination of these parameters with respect to a theoretical evolution.
It has been suggested to compare the distance between points which are respectively theoretical and measured, represented in a referential Is, Ic or still the lengths of the travelled paths measured.
It is, moreover, useful to have a reliable method for characterising the optical layers deposited. Then two possible aspects should be considered according to the usage made of the characterisation method. A dynamic aspect where after each new layer deposited of thickness dx, the layers deposited are characterised by measuring for instance optical signals S1 and S2. These signals enable then to get for instance the control parameter ∈ of the layers deposited. ∈ is the dielectric constant of the layer with ∈=n2 where n is the optical index. If ∈ is equal to the value ∈′ required, the following layer is deposited without re-adjusting the deposition parameters. Failing which, said parameters are adjusted in order to correct the error. The deposition parameters may thus be corrected in real time for optimised control of the deposition.
Such a method may also be used to characterise the evolution of the refraction index in relation to deposition parameters, without implementing any control of deposition. With reference to the fitting of the curve providing the variations of the dielectric constant ∈ as a function of these parameters, the parameters necessary to the production of a layer with a given index can be found. Such characterisation thus enables to minimise the number of cycles of deposition/characterisation necessary for the production of a layer with a given index.
Various direct digital reversal methods have thus been developed, but have prove suitable only for relatively thick films (200–500 Å). Others, based fitting methods seem more efficient, but have the disadvantage of requiring-tedious calculations and correction methods in order to stabilise the variation of the refraction index.
Approximations have however been suggested in order to simplify these calculations. It may be judicious, for instance, to reduce the number of parameters necessary to the fitting (“dispersion laws” [Heitz T and al.; J. Vac. Sci. Technol. A 18 (2000) 1303–1307], “Effective medium approximations” [Kildemo and al.; Applied Optics 37 (1998), 5145–5149]) or reduce, using suitable optical approximations the problems encountered when calculating the optical film (WKBJ, multiple integral methods, etc. [Kildemo and al.; Applied Optics 37, (1998) 113–124]).
However, these methods are too complex to be implemented in real time and in various situations such as those which are indeed encountered when producing the stacks of layers.
Besides, polynomial methods are known for reversing the ellipsometric signal [Lekner, J and al.; Applied Optics 33 (1994) 5159–5165; Drolet, J. P. and al.; Opt. Soc. Am. A 11 (1994) 3284–329]. These methods are, nevertheless, applicable only to non-absorbent monolayer and to samples exhibiting very simple structures. They use, moreover, the ellipsometric angles ψ and Δ as input parameters for the reversal formulae. Still, these values cannot be obtained directly by most ellipsometers.